I don’t believe this is a publicly released theme. It was created specifically for the Atom IDE announcement, and its creator is out of the office for a couple months so I can’t ask him if it’ll be released until he’s back.

EDIT: That said, it looks like the default one-light-ui theme, but with a couple of color variables tweaked to be more purple.

Sorry, the Atom IDE themes shown on the blog and site aren’t “real”. They are screenshots of One dark and One light and then adjusted with a color filter in Sketch to match the purple tone of Atom IDE.

I wanted the exact colors. So, I just edited the One Dark UI and syntax themes with my own css.
I just begun using Atom yesterday and wanted the purple theme to start off my experience. The modification that I made seems good enough for my own use.